Based on a trader's trading statistics, it is possible to make an estimate of risk of ruin. From looking at the proportion of winning trades/bets you make you can figure out P, x is entirely voluntary.

Symmetric risk of ruin (RoR):

     RoR = ((1 - P) / (1 + P))/x 

Where P is the probability of winning on any trade and;
x is the proportion of your account that you risk on any given trade.

This simple formula could be called the "symmetric risk of ruin" because it assumes you make as much money on each winning trade as you lose on each losing trade. The actual amount you win or lose per bet doesn't matter, just that the upside is the same as the downside, ie it is "symmetric". I'm leading up to the more complicated RoR formula that shows RoR for trades that are not symmetric.

If you look at the formula you will see that your risk of ruin increases greatly if you make big bets. Good traders aren't in the business to try to make a killing on each and every trade. They try to maximise their winning trades but they do this by holding onto winners throughout trends, not by making huge bets because they are confident in their own forecasting abilities.

If you chart RoR you will see that your chances of blowing it in just a few trades increase substantially when you make big bets.

The moral of this story is: don't be a cowboy, if you are trading for a living instead of punting you want to minimise risk of ruin, or maximise it's more upbeat complement, Probability of Success.

Your chances of being wiped out increase substantially if you risk too much per trade.